One known PSA process for the separation of oxygen from air employs a zeolite molecular sieve adsorbent which has the ability to adsorb nitrogen preferentially to oxygen and argon. In operation, a bed of this adsorbent is put through the following cycle: an adsorption step during which air is passed through the bed causing adsorption of most of the nitrogen and a portion of the argon and oxygen together with substantially all the carbon dioxide and water vapor, and production of an oxygen-rich product gas; a desorption step during which the bed outlet is closed, the bed is vented to atmospheric pressure through its inlet and is then evacuated through its inlet, so that the adsorbed gas are substantially removed, thereby regenerating it for the next adsorption step. In practice, two adsorbent beds are employed and operated on similar cycles but sequenced to be 180.degree. out of phase so that one bed is undergoing adsorption while the other bed is undergoing desorption. One disadvantage of the known process is that there is a waste of the energy employed in compressing the incoming air by virtue of venting the bed to atmospheric pressure during its regeneration. Another feature of the known process, which may be disadvantageous when the oxygen-rich product gas is administered to a patient in conjunction with an anesthetic, is that the product gas is also enriched in argon.
It is also known to produce a nitrogen-rich product by pressure swing adsorption utilizing a carbon molecular sieve that adsorbs oxygen more rapidly than nitrogen or argon. An example of a commercially used process, of this type is described in UK patent specification No. 2 042 365B. In this process, at the end of each adsorption and each regeneration step, the two beds are placed in communication so as to equalize the pressure therebetween. It is thus possible to conserve a portion of the energy that had been used to compress the incoming air.
The present invention relates to a method and apparatus for separating a gas mixture comprising at least three components which employs a plurality of discrete bed pressure reduction steps prior to placing the bed in communication with the atmosphere so as to regenerate it while also helping to reduce the impurity level of single component product gas.